I read more articles this past week about research on the proton. Some refined measurements. Some better insights into topics in quantum theory. Rather than add comments to related posts, I decided that a new post was appropriate. It struck me that the proton, as a composite particle (“particle” in the sense of an excitation or localized vibration in quantum fields), serves as a gateway into some key foundational questions of quantum physics. For example, interactions of fields associated with fermions (quarks within protons) and bosons (gluons binding quarks), and the dynamics of the quantum vacuum (frothy proton and foamy vacuum).
Quarks are the only elementary particles in the Standard Model of particle physics to experience all four fundamental interactions, also known as fundamental forces (electromagnetism, gravitation, strong interaction, and weak interaction), as well as the only known particles whose electric charges are not integer multiples of the elementary charge. – Wiki
1 > Inside the proton
This recent Symmetry Magazine article discusses an interesting quark-vacuum dynamic inside the proton. Constant quark interchange (swapping) and possibly detecting that dynamic due to chirality (handedness).
Symmetry Magazine > A joint Fermilab/SLAC publication > “Scientists search for origin of proton mass” by Sarah Charley (03/24/2020) – Only 1% of the mass of the proton comes from the Higgs field. ALICE scientists examine a process that could help explain the rest.
Protons are made up of fundamental particles called quarks and gluons. The quarks in protons are very light, and, as far as scientists know, gluons have no mass at all. Yet protons are much heavier than the combined masses of the three quarks they each contain.
“There is a lot of publicity about the origin of mass because of the Higgs boson,” says Dmitri Kharzeev, a nuclear theorist with a joint appointment at Stony Brook University and the Department of Energy’s Brookhaven National Laboratory. “But the Higgs is responsible for the mass of the quarks. The rest of it has a different origin.”
The quarks in protons are very light, accounting for only about 1% of the proton’s overall mass. The plausible—yet still unproven—theoretical explanation for this discrepancy is related to how quarks move through the vacuum.
This vacuum is not empty, says Sergei Voloshin, a professor at Wayne State University and a member of the ALICE experiment at CERN. The vacuum is actually filled with undulating fields that constantly burp particle-antiparticle pairs into and out of existence.
The three quarks that give protons their identity are forever jostling with these ethereal particle-antiparticle pairs. When one of these quarks gets too close to a vacuum-produced antiquark, it is annihilated and disappears in a burst of energy.
But the proton doesn’t wither and die when its quark is zapped out of existence; rather, the partner quark from the vacuum-produced particle-antiparticle pair steps in and takes the annihilated quark’s place …
[Question: Is this jostling and swapping dynamic only true for quarks? Any differentiation between UP and DOWN quarks? Is the swap rate related to gluon exchange? (Since quarks are confined by gluons, eh.)]
Scientists think that this incessant interchange of quarks is responsible for making a proton appear more massive than the sum of its quarks.
Because of the properties of the vacuum, the replacement quark will always have the opposite handedness from the original. That constant flipping of quarks from one handedness to the other is how theorists explain the majority of the proton’s mass.
Compare the above with what Wiki says – that 99% of the proton’s mass is from quantum chromodynamics (QCD) binding energy.
A modern perspective has a proton composed of the valence quarks (up, up, down), the gluons, and transitory pairs of sea quarks.
The rest masses of quarks contribute only about 1% of a proton’s mass. The remainder of a proton’s mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together.
In quantum chromodynamics, the modern theory of the nuclear force, most of the mass of protons and neutrons is explained by special relativity. The mass of a proton is about 80–100 times greater than the sum of the rest masses of the quarks that make it up, while the gluons have zero rest mass. The extra energy of the quarks and gluons in a region within a proton, as compared to the rest energy of the quarks alone in the QCD vacuum, accounts for almost 99% of the mass. The rest mass of a proton is, thus, the invariant mass of the system of moving quarks and gluons that make up the particle, and, in such systems, even the energy of massless particles is still measured as part of the rest mass of the system.
Two terms are used in referring to the mass of the quarks that make up protons: current quark mass refers to the mass of a quark by itself, while constituent quark mass refers to the current quark mass plus the mass of the gluon particle field surrounding the quark.
The internal dynamics of protons are complicated, because they are determined by the quarks’ exchanging gluons, and interacting with various vacuum condensates.
These recent calculations [for proton mass] are performed by massive supercomputers, and, as noted by Boffi and Pasquini: “a detailed description of the nucleon structure is still missing because … long-distance behavior requires a nonperturbative and/or numerical treatment…”
So, how is the unproven-theoretical explanation proposed by CERN ALICE scientists connected with the QCD model of binding energy? Are they competing or complimentary energy models?
Perhaps (as Wilczek might say), in either model the interactions slow down (or alter) the background fluctuations in the quantum vacuum and that reduced tempo is perceived as mass. In other words, the proton soup reveals the quantum vacuum as the primary reality from which our concept of mass arises.
So, the fabric of spacetime is layered: a space-time of extended fields and a roiling (temporal) vacuum. Feed energy into space and there’s a vast interplay of physical reality.
And as to why the proton (triquark) is revealing, Wilczek notes that “The best understood of these [space-filling] condensates consists of bound quark-antiquark pairs” – the quark-antiquark background.
 Well, mass as “bundled” energy (which has inertia). So, perhaps the stress–energy–momentum tensor of incessant (and confined) particle-antiparticle interactions – density and flux of energy and momentum (including gluons?) – with the quantum vacuum provides most of the proton’s mass.
Paraphrasing Wilczek, in QCD we can simply say that color charge (as in quarks) is the thing that gluons care about, eh. “The color charge of a quark creates a disturbance in the Grid — specifically, in the gluon fields … Disturbing the fields means putting them into a state of higher energy.”
Does the vacuum excite (“burp”) virtual particles in those fields? Or, do the conditions inside the proton stress the vacuum and “elicit” the (virtual) particles from the vacuum? In other words, do particles emerge from the vacuum when prodded in a certain way or …
Localized vibrations in fields (particles) may be characterized as quantum ocillators. What about fluctuations in the quantum vacuum? Unlocalized, random and chaotic? And if not ocillators, then what is the transition from nondescript “foam” to manifest virtual to instantiated real?
 Perhaps something similar for charge as well?
2 > Proton radius
This Quanta Magazine article from last year helped me better understand the Lamb shift and “proton radius puzzle.”
Quanta Magazine > “Physicists Finally Nail the Proton’s Size, and Hope Dies” by Natalie Wolchover (September 11, 2019) – A new measurement appears to have eliminated an anomaly that had captivated physicists for nearly a decade.
After Pohl’s muonic hydrogen result nine years ago [the muon-orbited protons to be 0.84 femtometers in radius], a team of physicists led by Eric Hessels of York University in Toronto set out to remeasure the proton in regular, “electronic” hydrogen. Finally, the results are in: Hessels and company have pegged the proton’s radius at 0.833 femtometers, give or take 0.01, a measurement exactly consistent with Pohl’s value. Both measurements are more precise than earlier attempts, and they suggest that the proton does not change size depending on context; rather, the old measurements using electronic hydrogen were wrong.
When an electron orbits the proton in the 2S state, it spends part of its time inside the proton (which is a constellation of elementary particles called quarks and gluons, with a lot of empty space). When the electron is inside the proton, the proton’s charge pulls the electron in opposing directions, partly canceling itself out. As a result, the amount of electrical attraction between the two decreases, reducing the energy that binds the atom together. The larger the proton, the more time the electron spends inside it, the less strongly bound the electron is, and the more easily it can hop away.
By firing a laser into a cloud of hydrogen gas, Hessels and his team caused electrons to jump from the 2S state to the 2P state, where the electron never overlaps the proton. Pinpointing the energy required for the electron to make this jump revealed how weakly bound it was in the 2S state, when residing partly inside the proton. This directly revealed the proton’s size.
Wiki’s section on proton charge radiius notes the new measurement using the Lamb shift vs. the more traditional method using electron scattering (and “root mean square” values).
Noting that an electron can spend some of its time inside the proton – that the 2S atomic orbital overlaps the proton’s but the 2P essentially does not – reveals the fuzzy dynamics of wave interference. And a surprisingly ample spatial stage (arena) even for a Planck-scale dance.
In his YouTube video on quarks, Don Lincoln notes that “Quarks are the particles that I’ve probably spent the most time studying so they may very well be my favorite.”
• YouTube > Fermilab > Don Lincoln > “Subatomic Stories: Quarks” (April 15, 2020).
Regarding analogies (vs. impenetrable math) on how “force” particles cause attraction and repulsion, this Fermilab video probably has the clearest visualizations you’ll find.
• YouTube > Fermilab > Don Lincoln > “Subatomic Stories: Forces the Feynman way” (May 6, 2020).
[Transcript quote] Basically, each of the forces is caused by a matter particle emitting a force particle that is then absorbed by another matter particle.
Taking the simple case of electromagnetism, for which the force carrying particle is a photon, two electrons experience the electromagnetic force when an electron shoots a photon at another electron, which absorbs it. This really isn’t all that hard to believe. That’s one way forces work in the familiar world.
[Animation] Imagine two people standing in boats and one of the boats has a heavy sack in it. If the person in the boat with a sack in it throws the sack, the boat will move in the opposite direction. Then, if a person in the other boat catches the sack, that boat will move too. This is kind of how the electric force will repel two objects with the same charge.
[Animation] People often ask how this analogy explains attractive forces. The version I use is one involving the same people in the same boats, but throwing a boomerang back and forth. One person throws the boomerang away from the other boat, but it circles around and the other person catches it. The result is that the boats move together. Of course, this is just an analogy and it isn’t perfect. My advice is to accept it if it helps you and if it doesn’t, I have a somewhat more accurate explanation.
Feynman’s explanation for how forces work is very much tied into quantum mechanics. Basically what he said is that when a photon travels from one charge to another, it can take literally any path, from the direct one, to one that is slightly indirect, to one that is truly bizarre … all paths must be considered. Then you use some complicated math and add them all up. If you have two charges of the same sign, the effect of adding up all of the paths results in a concentration of energy between them. Objects like to move to regions of lower energy, so the charges move away from one another.
[Animation] It’s like adding up the path of all the particles makes a hill between them, and the charges roll down the hill. If you do the same exercise with two charges of opposite sign, what happens is that the energy between the two charges goes down. Instead of a metaphorical hill between them, it’s a valley and the result is that the two charges again roll downhill, but this time they move together. That analogy is a little more accurate than the boat one, but to understand it in detail means you need to learn some very complicated math.
• QFT – How many fields are there?
If a field takes on a constant value through space and time, we don’t see anything at all; but when the field starts vibrating, we can observe those vibrations in the form of particles. — Carroll, Sean. The Big Picture: On the Origins of Life, Meaning, and the Universe Itself (p. 174). Penguin Publishing Group. Kindle Edition.
• The proton and perturbation problem
The proton, which in its simplest description is three quarks and some gluons, gets non-perturbative when you look a little closer. Quark and antiquark pairs appear from the vacuum, gluons are emitted and absorbed, and the result, says Shanahan, is a “bubbling, boiling, dynamically complicated structure.”
Perimeter Institute #4 > Phiala Shanahan Public Lecture: The Building Blocks of the Universe
Proton classification: Fermions (so-called matter particles) > hadrons (quark-based particles – bound states of their “valence quarks” and antiquarks) > baryons > nucleons (triquarks) > protons.
“It is not uncommon to hear that energy is ‘equivalent’ to mass. It would be more accurate to state that every energy has an inertia and gravity equivalent, and because mass is a form of energy, then mass too has inertia and gravity associated with it.” – Wiki
Fermions or so-called matter particles resist confinement (Planck-level bunching). They obey the Pauli exclusion principle, unlike bosons. Like charged fermions repel each other. In that sense, fermions “take up space,” although the size of elementary/fundamantal fermions is an open question.
Anti-fermions annihilate fermions, typically producing photons (in various parts of the electromagnetic spectrum). Photons (as elementary bosons) are their own antiparticles and can “pile on top of one another” as in lasers (while other identical composite bosons can crowd into exotic quantum condensates).
Language is tricky when it comes to quantum physics – something that I’ve mentioned elsewhere. So, in writing this post, I realized that characterizing bosons as “force carriers” may be a metaphorical rabbit hole. Somewhat a conceptual dead end – something which proton structure (and dynamics) maybe reveals.
Wiki > Quark
Quarks are spin-1⁄2 particles, implying that they are fermions according to the spin–statistics theorem. They are subject to the Pauli exclusion principle, which states that no two identical fermions can simultaneously occupy the same quantum state. This is in contrast to bosons (particles with integer spin), of which any number can be in the same state. Unlike leptons, quarks possess color charge, which causes them to engage in the strong interaction. The resulting attraction between different quarks causes the formation of composite particles known as hadrons.
The quarks that determine the quantum numbers of hadrons are called valence quarks; apart from these, any hadron may contain an indefinite number of virtual “sea” quarks, antiquarks, and gluons, which do not influence its quantum numbers.
Wiki > Sea quarks
Sea quarks are virtual quark–antiquark (qq-bar) pairs. Sea quarks form when a gluon of the hadron’s color field splits; this process also works in reverse in that the annihilation of two sea quarks produces a gluon. The result is a constant flux of gluon splits and creations colloquially known as “the sea”. Sea quarks are much less stable than their valence counterparts, and they typically annihilate each other within the interior of the hadron. Despite this, sea quarks can hadronize into baryonic or mesonic particles under certain circumstances.
In the standard framework of particle interactions (part of a more general formulation known as perturbation theory), gluons are constantly exchanged between quarks through a virtual emission and absorption process. When a gluon is transferred between quarks, a color change occurs in both; for example, if a red quark emits a red–antigreen gluon, it becomes green, and if a green quark absorbs a red–antigreen gluon, it becomes red. Therefore, while each quark’s color constantly changes, their strong interaction is preserved.
So, the unproven-theoretical explanation proposed by CERN ALICE scientists regarding constant flipping of quarks drives preservation of their strong interaction – or binding energy, which is most of the proton’s mass?
11 thoughts on “Proton soup – a turbulent, dynamically complicated structure”
Quanta Magazine > “Physicists Peer Inside a Fireball of Quantum Matter” by Charlie Wood (July 30, 2019) – Experimenters in Germany have glimpsed the kind of strange, non-atomic matter thought to fill the cores of merging neutron stars.
Using an indirect computational method of examining the outcomes of collider experiments, the High Acceptance DiElectron Spectrometer (HADES) collaboration confirmed a “fireball” of particles corresponded to an exotic state of matter.
The Quanta Magazine article cited above (“Physicists Finally Nail the Proton’s Size, and Hope Dies” – September 11, 2019) noted that: “When an electron orbits the proton in the 2S state, it spends part of its time inside the proton.”
Because electron orbitals can extend not just into an atom’s nucleus but inside its nucleons, I was reminded of this previously cited Gizmodo article about double electron capture:
“Dark Matter Detector Makes Incredible Neutrino Observation” (4-25-2019).
Wiki > Double electron capture
This article summarizes some research on the dynamics of proton soup – “the dance of quantum activity that includes virtual quarks … in the hubbub of virtual activity that surrounds all particles.” Feynman diagrams (from QED) are noted as a common mathematical tool to explore a particle’s “virtual entourage,” but may be questionable for QCD. Perhaps in part because, unlike photons, gluons can “trip over themselves” (in the gluon field).
Quanta Magazine > “What Goes On in a Proton? Quark Math Still Conflicts With Experiments” by Charlie Wood (May 6, 2020): Two ways of approximating the ultra-complicated math that governs quark particles have recently come into conflict, leaving physicists unsure what their decades-old theory predicts.
This Symmetry article contains one of the best lay descriptions of what happens when protons collide at nearly the speed of light. I’m still puzzled by how “throwing bosons back and forth” binds “particles” together – they carry a “stick together” message. I’d prefer an explanation using quantum field theory. As well as how colliding energetic gluons – field excitations – transform into other field excitations via “the wonders of quantum mechanics.” Fourier transform visualizations?
Symmetry Magazine > “The large boson-boson collider” by Sarah Charley (04/30/2020).
So, stable matter depends on stable protons, a “soup” held together by quantum chromodynamics binding energy. The force is strong, eh. A complex dynamic quantum state which is the subject of active research.
Then our everyday experience depends on the stability of nuclides, a composite of quantum states of nucleons, held together by nuclear binding energy. Yet another “Goldilocks” energy level for the dance of localized field excitations.
But that medley can be jiggered. A proton (or neutron) can “drip” from a nucleus. (Find a visualization for that?)
This Phys.org article discusses research that created atoms of exotic fluorine 25 (atomic number 9) from oxygen atoms (atomic number 8) by adding a proton. What caught my attention was that adding and then removing the extra proton did not restore the oxygen atom to its original quantum state: “the oxygen 24 at the core of the fluorine isotope mostly existed in excited states quite different from oxygen 24 itself.”
Energy state(Oxygen 24) + Energy state(proton) = Energy state(Fluorine 25)
Energy state(Fluorine 25 – proton) ≠ Energy state(Oxygen 24)
Phys.org > “A single proton can make a world of difference” by Riken (May 28, 2020).
 Wiki: According to Byrne, stable nuclides are defined as those having a half-life greater than 10^18 years. A common example of an unstable nuclide is carbon-14 that decays by beta decay into nitrogen-14 with a half-life of about 5,730 years.
The boundaries of the valley [valley of stability] correspond to the nuclear drip lines, where nuclides become so unstable they emit single protons or single neutrons.
Wiki: “An arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus.”
Drip lines are defined for protons and neutrons at the extreme of the proton-to-neutron ratio; at p:n ratios at or beyond the drip lines, no bound nuclei can exist.
Protons are hadrons, composite particles made of quarks. But there are many other hadrons. So, why are protons and neutrons the only two that constitute our everyday world? Well, a stable world depends on stable particles, like the proton.
This article contains a brief historical recap on looking to the skies for sources of radiation. And later the development of the quark model.
• Symmetry Magazine > “Hundreds of hadrons” by Jerald Pinson (June 30, 2020) – Hadrons count among their number the familiar protons and neutrons that make up our atoms, but they are much more than that.
Adding another decimal place to QED’s accuracy … 13 decimal places. Measurements on ordinary hydrogen vs. muonic hydrogen. Doppler-free two-photon frequency comb spectroscopy: pulsed vs. continuous wave laser and spectral range research.
“The new result however suggests that the problem is of experimental rather than fundamental nature.”
• Phys.org > “Unprecedented accuracy in quantum electrodynamics: Giant leap toward solving proton charge radius puzzle” by Katharina Jarrah, Max Planck Society (Nov 27, 2020)
A further take on proton structure by Ethan Siegel.
Forbes > “What’s Really Inside A Proton?” by Ethan Siegel Senior Contributor (Feb 15, 2021)
If proton structure was effectively symmetric, then protons would lack charge. And for want of a charge, all matter be lost.
• Quanta Magazine > “Decades-Long Quest Reveals Details of the Proton’s Inner Antimatter” by Natalie Wolchover (February 24, 2021) – Twenty years ago, physicists set out to investigate a mysterious asymmetry in the proton’s interior. Their results, published today, show how antimatter helps stabilize every atom’s core.
Article has interesting visualization of the proton soup / sea of quarks and antiquarks. Fluid dynamics. (Reminds me of the boiling swirl of rice and water in a glass bowl cooking in my microwave. Energy feeding a mixture in a confined space.) Also more photos of the lab.
Pion cloud model vs. statistical (gas) model [a distribution of speeds that depend on whether they possess integer or half-integer amounts of angular momentum] … QCD … self-dealing gluons … unsolvable equations … proton fluctuation between neutron-pion pair (toss a basketball to itself) … the antiquark’s momentum fraction … proton spin [bookkeeping or spin budget] …. SpinQuest …
What’s at stake:
Here’s another take on the Fermilab experiment.
• Argonne National Lab > “Nature’s funhouse mirror: understanding asymmetry in the proton” by Savannah Mitchem (February 24, 2021) – Asymmetry [broken symmetry] in the proton confounds physicists, but a new discovery [an experiment at Fermilab] may bring back old theories to explain it.
How do you squeeze or stretch a proton and detect such deformation?
• Phys.org > “How stiff is the proton?” by US Department of Energy (October 3, 2022)
Effective Field Theories (EFTs)
Quantum chromodynamics (QCD)
Proton Compton scattering
High Intensity Gamma Ray Source (HIGS)
Credit: Pixabay/CC0 Public Domain
Here’s another case of thinking like a physicist – a scheme for describing a complicated quantum system in terms of a simpler one – when studying “low-energy” interactions. An application of perturbation theory. And years of analysis to validate those theoretical predictions.
• Phys.org > “The direct measurement of a proton’s generalized polarizabilities in the strong quantum chromodynamics regime” by Ingrid Fadelli (November 4, 2022) – Direct tests of QCD are notoriously hard to come by.
How exactly do the spin-dependent properties of the proton arise?
Credit: Jacek rybak, CC BY-SA 4.0, via Wikimedia Commons
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